Many image systems exist that employ local or remote non-destructive sensing of spectrally dependent properties of substances for identifying the presence of one or more substances. These substances could include, for example, gases, chemicals, minerals, pollutants, and objects both man-made and natural in the form of a gas, liquid or solid. Examples of local sensing and identifying methods that have been used to detect spectral properties include ion mobility spectrometry, flame photometry, mass spectrometry, electrochemistry, detection paper methods, surface acoustic wave methods, laser-induced breakdown spectroscopy, photo ionization detection, gas chromatography and cavity-ring-down spectroscopy. Local sensing and identifying systems and methods are limited by the ability to retrieve a sample or locate detection equipment near a target area.
Examples of conventional remote detectors include Raman spectroscopy, forward looking infrared (FLIR) detectors and differential absorption light detection and ranging (LiDAR) (DIAL) detectors. Raman based sensors illuminate samples with ultraviolet light and look for a Raman shift in the reflected signal. Unfortunately, the absorption of the atmosphere strongly absorbs the ultraviolet light severely limiting the range and sensitivity of such systems. Furthermore, only a limited number of chemical compounds can be detected. A FLIR sensor uses a FLIR detector array and a set of filters that allows a user to visually detect the presence of certain substances. The sensitivity and selectivity of FLIR detection are highly dependent on a user's ability to interpret contrasts created in the visual field by looking at a scene using various different filters. Such FLIR based systems are frequently referred to as HyperSpectral Imagers (HIS) or MultiSpectral Imagers. Many DIAL sensors use carbon dioxide lasers to identify chemical substances. One drawback of carbon dioxide lasers is that they are limited to using the spectral lines available from the carbon dioxide gain media. This limited wavelength selection limits the sensitivity and selectivity of prior art DIAL sensors.